the fact that is defined does not mean that i can't change it, it's like buying a blue car and saying that you can't re-paint it with a red color. For what i know this is possible in theory, also the language used in a pdf is more like an interpreted language and the pdf reader is an interpreter .
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axisOct 7 '12 at 15:30

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Well that analogy isn't entirely accurate cause you don't want to change the PDF file, right? So it's more like buying a car and wanting it to be red for you but blue for everyone else. The same question has been asked here, here and here; the answers are pretty much the same as mine.
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duxk.ghOct 7 '12 at 21:12